Thermal spray (TS) coating technology is one of many methods for applying coatings for the protection of materials in harsh environments. Thermal spray coating technology historically has been associated with the application of thick coatings whose bulk material properties allow the substrate to perform in a manner beyond its capabilities when used in an uncoated condition. The desire to apply coatings with diverse thermophysical properties has resulted in a diverse family of coating processes sharing the same basic elements. In general, a source of heat is used to generate a region of high enthalpy, and a material, originally in powder, wire, or rod form, is introduced into the region to form droplets in either a molten or plastic state. These droplets are accelerated onto a prepared substrate where they bond to form a protective coating. Some of the limitations of the thermal spray process generally are observed to be relatively low coating-to-substrate bond strengths, a high amount of connective porosity within the coating, and high thickness of the coating. TS coatings are generally sprayed to a thickness that overcomes the connective porosity. Fine, closed, dispersed porosity is advantageous for ceramic coatings since such porosity introduces thermal shock resistance and permits a reduced coating thickness.
The structure of thermal sprayed coatings typically is lamellar in nature comprising particles which have been flattened by impingement at high particle temperature onto the substrate. By far the most versatile thermal spray process is the plasma spray process. An outgrowth of research into reentry phenomena for the space program, plasma spraying has resulted in the ability to apply coatings comprising high melting point materials, such as ceramic materials. Such plasma spray processes include DC/AC plasma torches (either transferred or non-transferred arc) and radio frequency (RF) plasma torches.
A DC non-transferred arc plasma torch involves maintaining an electric arc within the torch and not involving the substrate as part of the electric arc-generating circuit. In particular, a stick cathode, generally fabricated of thoriated tungsten, is coaxially aligned with a ring anode/nozzle generally fabricated of OFHC copper or, in some torches, with an additional lining of tungsten. An electrical potential is imposed between the anode and the cathode. Gas to be ionized flows around the cathode and through the anode/nozzle. The gap between the cathode and the anode/nozzle is broken down by applying an overvoltage which ionizes the gas and allows electrical current to flow between the electrodes. Particles to be melted are injected radially into the plasma plume either just prior to the exit end of the anode/nozzle (i.e. particle lip feed) or external of the exit end of the anode/nozzle (particle external feed).
Primary plasma gases are typically argon and nitrogen. Secondary gases are typically hydrogen and/or helium. In some infrequent applications, argon and nitrogen are used as the plasma gases. For diatomic gases, such as nitrogen and hydrogen, the gases are dissociated and then ionized. For monatomic gases, such as argon and helium, the gases are substantially, if not totally, ionized. In the plasma plume, ionized gases return to lower energy levels, and the gases which were initially diatomic (i.e nitrogen and hydrogen) recombine with a resulting release of large quantities of heat over a narrow temperature range, thereby heating the particles.
In the plasma spray devices described hereabove, the electrode material can contaminate the coatings being applied. When high purity coatings are needed, RF (radio frequency) plasma generators are used. The high frequency plasma generator operates without electrodes and thus yields an uncontaminated plasma and resultant coating. The RF plasma generator is a simple device wherein gas flows through a non-electrically conductive tube closed at the top and open at the bottom and surrounded by a high frequency coil. Ionized gas required for starting the plasma is produced by the introduction of a carbon or tungsten rod into the working space of the coil. By coupling to the starter rod, the high frequency generator heats the rod until gas ionization takes place around the rod. With the high frequency generator now coupled to the ionized gas, the starter rod is removed and plasma gas flowed into the plasma.
An object of the present invention is to provide a plasma spray apparatus and method for applying a coating in a manner to improve coating-to-substrate bond strength and improve coating density (substantially reduced interconnected coating porosity and defects) at reduced coating thicknesses.
Another object of the present invention is to provide a capability to control closed porosity in ceramic and cermet coatings applied by plasma spraying.
Still another object of the present invention is to provide a highly dense, adherent ceramic, metal, and cermet coating applied on substrates at reduced cost by plasma spraying.